Metal ion-induced dual fluorescent change for aza-crown ether acridinedione-functionalized gold nanorods and quantum dots

Article abstract of DOI:10.1039/c2nj40444c

Aza-crown ether acridinedione-functionalized quantum dots (ACEADD-QDs) and aza-crown ether acridinedione-functionalized gold nanorods (ACEADD-GNRs) have been developed as a pair for a fluorescent chemosensor detecting metal ions. The ACEADD-QDs have dual emissions at a visible wavelength of ～430 nm from the acridinedione dye moiety and at a near-infrared (NIR) wavelength of ～775 nm from the CdTeSe QDs. In the presence of Ca²⁺ or Mg²⁺ ions, the ACEADD-QD and ACEADD-GNR pair can form a sandwich complex mediated by the metal ion. The ACEADD-QD and ACEADD-GNR complex pair shows visible fluorescence enhancement from the acridinedione dye and concurrent fluorescence quenching from the NIR QD. The aza-crown ether complex results in the suppression of photoinduced electron transfer from the aza-crown ether to the acridinedione dye moiety. At the same time, the QD fluorescence can be effectively quenched by the nanometal surface energy transfer from the QD to the GNR. This ACEADD-QD and ACEADD-GNR pair can effectively transduce the selective binding event of crown ethers with metal ions into the simultaneous modulation of the enhancement in dye fluorescence and the quenching of QD emission, which can open a new strategy for ratiometric sensors that are selective and robust against the environment conditions.

Full text of DOI:10.1039/c2nj40444c

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LETTER
Metal ion-induced dual fluorescent change for aza-crown ether
acridinedione-functionalized gold nanorods and quantum dotsw
a
a
Ranganathan Velu,z Nayoun Won,z Jungheon Kwag,b Sungwook Jung,b
Jaehyun Hur,c Sungjee Kim*ab and Nokyoung Park*c
Received (in Montpellier, France) 29th May 2012, Accepted 8th July 2012
DOI: 10.1039/c2nj40444c
Aza-crown ether acridinedione-functionalized quantum dots
(ACEADD-QDs) and aza-crown ether acridinedione-functionalized
gold nanorods (ACEADD-GNRs) have been developed as a pair for
a fluorescent chemosensor detecting metal ions. The ACEADD-QDs
have dual emissions at a visible wavelength of B430 nm from the
acridinedione dye moiety and at a near-infrared (NIR) wavelength of
B775 nm from the CdTeSe QDs. In the presence of Ca2+ or Mg2+
ions, the ACEADD-QD and ACEADD-GNR pair can form a
sandwich complex mediated by the metal ion. The ACEADD-QD
and ACEADD-GNR complex pair shows visible fluorescence
enhancement from the acridinedione dye and concurrent fluores-
cence quenching from the NIR QD. The aza-crown ether
complex results in the suppression of photoinduced electron
transfer from the aza-crown ether to the acridinedione dye
moiety. At the same time, the QD fluorescence can be effectively
quenched by the nanometal surface energy transfer from the QD
to the GNR. This ACEADD-QD and ACEADD-GNR pair can
effectively transduce the selective binding event of crown ethers with
metal ions into the simultaneous modulation of the enhancement in
dye fluorescence and the quenching of QD emission, which can
open a new strategy for ratiometric sensors that are selective and
robust against the environment conditions.
emitters, and other optoelectronics.1–5 For example, many
biosensors have been developed using photoluminescence
modulations through fluorescence resonance energy transfer
(FRET) between a donor and acceptor pair.6–9 Metallic
surfaces or metal nanoparticles have been actively studied
for the dipole-surface energy transfer and the induction of
fluorescent probe quenching.10–13 Such donor–acceptor type
fluorescent sensors are advantageous due to their high sensitivity,
but may be influenced by many factors including photobleaching,
analyte concentration, environmental changes (pH, polarity,
temperature, and etc.), and instability under illumination. In
comparison to simple luminescence-based detection, ratiometric
sensors can afford simultaneous recording of two emission
intensities at different wavelengths in the presence or absence
of the analytes and thus can permit signal rationing and thus
increase the dynamic range and provide built-in correction for
the environmental effects.14,15
Based on the well-known binding ability of crown ethers to
host s-block metals, many fluorescent chemosensors have been
designed in the recent years for alkali and alkaline earth ions
containing at least one crown ether moiety (or a related derivative)
which behaves as a ‘recognition subunit’ connected to one or more
fluorophores.16–18 The efficiencies of energy transfer processes are
related to the spectral overlap of the acceptor’s absorption with the
donor’s emission, and sensitively depend on the distance between
the two molecules.19 In the present work, we have developed
aza-crown ether acridinedione-functionalized gold nanorods
(ACEADD-GNRs, 1) and aza-crown ether acridinedione-
functionalized quantum dots (ACEADD-QDs, 2) towards
specific sensors for Ca2+ and Mg2+ (Scheme 1).
Energy transfer in nanoscale donor–acceptor systems has extensive
applications in sensors, bioimaging, photovoltaic devices, light
a Department of Chemistry, Pohang University of Science &
Technology, Pohang, 790-784, Republic of Korea.
E-mail: sungjee@postech.ac.kr; Fax: +82-54-279-1498;
Tel: +82-54-279-2108
b School of Interdisciplinary Bioscience and Bioengineering,
Pohang University of Science & Technology, Pohang, 790-784,
Republic of Korea
The ACEADD-QDs have dual emissions at a visible wavelength
of B430 nm from the acridinedione (ADD) dye moiety and at a
near-infrared (NIR) wavelength of B775 nm from the CdTeSe
QDs. QDs emitting at the NIR range are judiciously chosen for the
efficient energy transfer to GNRs which has high extinction at the
NIR range. The CdTeSe QD emission has been judiciously chosen
for the maximal overlap with the longitudinal plasmon extinction
of the GNR to achieve the efficient energy transfer from the
QD to GNR.20 The pair of 1 and 2 can effectively transduce
the selective binding event of crown ether with metal ions
into the simultaneous modulation of enhancement in ADD
c Samsung Advanced Institute of Technology, Mt. 14-1,
Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446-712,
Republic of Korea. E-mail: n2010.park@samsung.com
w Electronic supplementary information (ESI) available: Experimental
details for the aza-crown ether acridinedione synthesis, PL spectra of
the mixture of aza-crown ether acridinedione-functionalized gold
nanorods and aza-crown ether acridinedione-functionalized quantum
dots upon the addition of Ca2+, Mg2+, Na+, K+ or Ba2+ ions, PL
spectra of aza-crown ether acridinedione-functionalized quantum dots
upon the addition of Ca2+ ions, and Stern–Volmer plots. See DOI:
10.1039/c2nj40444c
z These authors contributed equally to this work.
c
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Scheme 1 Schematic structures of aza-crown ether acridinedione-
functionalized gold nanorods (ACEADD-GNRs, 1) and aza-crown
ether acridinedione-functionalized quantum dots (ACEADD-QDs, 2)
and illustration of fluorescence change of the complex (3) upon the
addition of metal ions.
dye fluorescence and quenching of the QD emission. When
aza-crown ether (ACE) is conjugated to ADD dye, photo-
induced electron transfer (PET) occurs from the nitrogen lone
pair of the ACE to the relatively electron deficient ADD
moiety inducing the fluorescence quenching of the ADD. As
a ‘recognition subunit’, the ACE can bind metal ions and PET
is suppressed resulting in the recovery of the ADD fluores-
cence. At the same time, ACE can form the sandwich complex
in the presence of cations and can result in the formation of
ACEADD-QDs and ACEADD-GNRs complex pair 3. The
efficiency of energy transfer is critically related to the distance
between the donor and the acceptor. The decreased distance
between the QD and GNR in 3 leads to the efficient energy
transfer from QDs to GNRs and results in the QD fluores-
cence quenching. Studies for Ca2+ sensors have been recently
reported by various groups.21 Ca2+ specific sensors using the
principle of PET was suggested by de Silva and Gunaratne.22
However, the chemosensors reported so far have been
mostly achieved by charge or energy transfer processes in
chromophores.23,24 In addition, to the best of our knowledge,
s-block metal ion sensing based on dual fluorescence modulations
in visible and NIR wavelengths has not been reported.
Fig. 1 (a) Absorption spectra of aza-crown ether acridinedione-
functionalized gold nanorods upon the addition of 0–85 mM of
Ca2+ ions in acetonitrile. (b) Absorption and photoluminescence
spectra of CdTeSe quantum dots in hexanes.
of B430 nm and from the CdTeSe QD at a NIR wavelength of
B775 nm under the excitation at 360 nm. The ACEADD
showed marginal difference in the photoluminescence property
after the conjugation with QD, which manifest the negligible
FRET from the dye to the QD. Using the excitation wavelength
at 500 nm, the ACEADD-QD sample showed the emission
from QD only.
A mixture of 1 and 2 retained the optical properties of each,
with the emission maxima at 430 nm from the ADD moiety
and at 775 nm from 2. Upon the addition of Ca2+ ions,
fluorescence from the ADD was enhanced as previously
reported (Fig. 2a).29 Upon increasing the Ca2+ ion concen-
tration from 8 to 80 mM, the emission at 430 nm from the
ADD moiety was gradually increased by up to 2.2 fold from
the initial intensity. In the absence of cations, PET occurs from
the N-atom of the cation receptor (1-aza-15-crown-5) to
ADD, inducing the fluorescence quenching. In the presence
of cations which can complex to the aza-crown ether, fluores-
cence of ADD can recover by the PET suppression. On the
other hand, fluorescence from the QDs was quenched by the
addition of Ca2+ ions (Fig. 2b). Upon increasing Ca2+ ion
concentrations from 8 to 80 mM, the quenching efficiency
for the QD emission was linearly increased by up to 80%,
which suggests the high FRET efficiency over 80%. The dual
emission changes at visible wavelength and NIR wavelength
can be represented by the emission intensity ratios. Fig. 2c
shows the emission intensity ratios between ADD emission at
430 nm and QD emission at 775 nm. The ratio was increased
linearly with increasing Ca2+ concentration. This suggests
that our ACEADD-QD and ACEADD-GNR pair based
sensor functions well for the ratiometric detection of Ca2+
ions, which can potentially alleviate interferences from external
environments. The QD fluorescence quenching is attributed to
The ADD in acetonitrile has the absorption peak at 360 nm
and emission peak at 430 nm.25,26 The peak at 360 nm is
attributed to the intramolecular charge transfer from the
nitrogen to the carbonyl oxygen in the ADD moiety and the
emission at 430 nm to that of the local excited (LE) state.
Fig. 1a shows absorption spectra of 1 in acetonitrile. The
peaks around 515 and 770 nm are originated from the
transverse and longitudinal surface plasmon resonances of
GNRs. Upon the addition of Ca2+ ions, red-shift of the
longitudinal peak was observed from 770 nm continuously
up to 920 nm as the ion concentration increased, which is due
to the inter-GNR plasmon mode coupling from the linearly
linked GNRs (Fig. 1a). The QDs were prepared using
previously reported methods.27,28 Absorption and emission
spectra of CdTeSe QDs are shown in Fig. 1b. The QDs have
the excitonic peak at 740 nm and emission peak at 775 nm. As
can be seen from Fig. 1a and b, the CdTeSe QDs have been
chosen so that the emission of the CdTeSe QDs overlaps well
with the strong longitudinal plasmon resonance of GNRs.
Absorption and photoluminescence spectra of ACEADD and
ACEADD-QDs are shown in Fig. S1 (ESIw). The ACEADD-QDs
show dual emissions from the ADD dye at a visible wavelength
c
1726 New J. Chem., 2012, 36, 1725–1728
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Fig. 2 Photoluminescence (PL) spectra of the mixture of aza-crown ether acridinedione-functionalized gold nanorods (460 pM) and aza-crown ether
acridinedione-functionalized quantum dots (920 pM) upon the addition of 0–80 mM of Ca2+ ion in acetonitrile. (a) Emission from the acridinedione
moieties (lex = 360 nm). (b) Emission from the CdTeSe quantum dots (lex = 500 nm). (c) Ratio of PL intensity at 430 nm to PL intensity at 775 nm.
the increased energy transfer from the QDs to GNRs in closer
proximity. The proximity between the QD and GNR resulted
from the sandwich formation of 3 mediated by the sandwiched
cation–crown ether complex. The formation of 3 was further
confirmed by HR-TEM images (Fig. 3). The TEM sample of 3
was prepared by drop-casting a dilute suspension on a carbon-
coated copper grid. QDs were found to have an average
diameter of 7.0 nm. In the absence of cation, the free
ACEADD-QDs and ACEADD-GNRs were well-dispersed
and aggregates were not observed (Fig. 3a). When 80 mM of
Ca2+ ions were added to the mixture of 1 and 2, the distance
between nanoparticles became closer through the sandwich
complex between ACEADDs. As a result, QDs gathered
densely around the edges of GNRs (Fig. 3b). The decreased
distances between QDs and GNRs enhanced the efficient energy
transfer from QDs to GNRs and subsequent quenching of the
QD emission. Our ACEADD-QD and ACEADD-GNR pair
based cation sensor also showed the fluorescence modulation in
the presence of Mg2+ ions (Fig. S2, ESIw). Similar to the case of
Ca2+ ion, the fluorescence from the ADD moiety was gradually
enhanced and the QD emission was simultaneously quenched as
the concentration of Mg2+ ions increased from 8 to 80 mM.
A control experiment was performed by adding Ca2+ ions
to 2 in the absence of 1. No noticeable change was found in the
QD fluorescence, excluding possibilities of QD fluorescence
change by direct interactions between the QDs (or QD surfaces)
and Ca2+ ions (Fig. S3, ESIw). The quenching efficiencies on
Ca2+ or Mg2+ ions were studied using the Stern–Volmer plot.
Stern–Volmer constants (KSV) were obtained for Ca2+ and
Mg2+ ions as 47300 Mꢀ1 and 44400 Mꢀ1, respectively
(Fig. S4, ESIw). Other metal ions such as Na+, K+ and Ba2+
were also added to the mixture of 1 and 2. No noticeable change
in the absorption or emission was found, indicating the absence
of complex 3 formation (Fig. S5, ESIw).
The reversible nature of our system was confirmed by the
addition of ethylenediaminetetraacetic acid (EDTA) to complex
3 which was formed by the addition of 85 mM of Ca2+ ions to the
mixture of 1 and 2. The fluorescence from ADD was enhanced
about 2 fold by the addition of Ca2+ ions and gradually decreased
by increasing the EDTA concentration up to 95 mM (Fig. 4a). The
fluorescence decrease is attributed to the detachment of Ca2+ ions
from crown ether and subsequent recurrence of PET. At the same
time, the QD fluorescence was quenched with the addition of
Ca2+ ions and gradually recovered by increasing the EDTA
concentration up to 95 mM (Fig. 4b). The QD fluorescence
recovery results from the increased distance between 1 and 2.
In summary, we have demonstrated a fluorescent chemo-
sensor for metal ions based on a pair of ACEADD-GNR and
ACEADD-QD. The sensor shows excellent selectivity towards
Ca2+ and Mg2+ ions resulting in the enhanced fluorescence from
ADD in the visible range which is attributed to the PET suppres-
sion and concurrent QD fluorescence quenching at the NIR range
which is due to the energy transfer from QD to GNR. In the
successive work, the surface-coating of GNRs can be modified with
multilayer polyelectrolytes for the improved biocompatibility for
various biological applications.30 As for the potential QD toxicity
issue, the CdTeSe QDs are desired to be replaced by QDs that
consist of less toxic elements such as InP or CuInS2 based
QDs.31–33 Our ACEADD-QD and ACEADD-GNR pair system
can be a prototype for a general dual fluorescence modulation
(turn-on at the visible range and turn-off at NIR) sensor for robust
and sensitive sensing applications.
Experimental
Materials
All the metal ions used in this assay were in the form of
their perchlorates. Gold(III) chloride hydrate (HAuCl4ꢁ3H2O,
Z 99.9%), silver nitrate (AgNO3, Z 99.9%), L-ascorbic acid,
sodium borohydride (NaBH4, 99%), cetyl trimethylammonium
bromide (CTAB, 98%), and polyvinylpyrrolidone (PVP) were
purchased from Sigma-Aldrich.
Fig. 3 (a) HR-TEM image of the mixture of aza-crown ether acridi-
nedione-functionalized gold nanorods (aspect ratio of 3.7) and aza-
crown ether acridinedione-functionalized quantum dots. (b) HR-TEM
image of the mixture after the addition of 80 mM of Ca2+ ions.
c
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New J. Chem., 2012, 36, 1725–1728 1727

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Fig. 4 Photoluminescence (PL) spectra of the mixture of aza-crown
ether acridinedione-functionalized gold nanorods (460 pM) and
aza-crown ether acridinedione-functionalized quantum dots (920 pM)
which was previously treated with 85 mM of Ca2+ ions in acetonitrile.
0–95 mM of ethylenediaminetetraacetic acid (EDTA) were added to the
solution. (a) Emission from the acridinedione moieties. (b) Emission
from the CdTeSe quantum dots.
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The ACEADD was synthesized by our previously reported
method (synthetic details and characterization data are available
in the ESIw).29 The GNRs used in this study were synthesized by a
surfactant-stabilized, seedless one-step technique following a
procedure reported in the literature.34,35 HAuCl4 (20 mM),
AgNO3 (0.1 mM), and ascorbic acid (0.1 M) were mixed in
5 mL of 0.1 M aqueous CTAB solution. 20 mL of 1.6 mM NaBH4
was used as the reducing agent to generate GNRs. The GNRs
were further stabilized by adding PVP. They were centrifuged and
re-dispersed in ethanol by adapting a procedure utilized by Liz-
Marzan and coworkers.36,37 25 mM of ACEADD was added to
the PVP-stabilized GNR solutions under stirring and the solution
was kept undisturbed for 24 h to allow adequate time for the self-
assembly of the ACEADD monolayer onto the ends of the GNR.
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This work was supported by a Korea Science and Engineering
Foundation grant funded by Ministry of Science and Technology
(20110018601), the Korea Health 21 R&D Project Ministry of
Health & Welfare (A101626), the Priority Research Center
Program through National Research Foundation of Korea
(NRF) (2011-0031405 and 20110027727), the Ministry of Educa-
tion, Science and Technology through NRF (20110019635), the
Basic Science Research Programs (20110027236), and Defense
Acquisition Program Administration and Agency for Defense
Development under the contract (ADD-10-70-06-02).
c
1728 New J. Chem., 2012, 36, 1725–1728
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012